A magnetic resonance imaging apparatus (hereafter referred to as “MRI apparatus”) is an apparatus that uses a nuclear magnetic resonance (hereafter referred to as “NMR”) phenomenon that occurs when a subject (living body) placed in a uniform static magnetic field is irradiated with high frequency pulses. The MRI apparatus measures the NMR signal generated by the nucleus spin of the atoms constituting the tissue of the subject. The MRI apparatus images the head, abdomen, limbs, etc. of the subject two-dimensionally or three-dimensionally using the measured NMR signal. During imaging, positional information, being different depending on a gradient magnetic field, is given to the NMR signal, and the NMR signal is measured as a digital signal. The measured digital signal is subjected to predetermined signal processing and a tomographic image is generated.
In such an MRI apparatus, the enhancement of the static magnetic field intensity thereof is underway to improve the spatial resolution and sensitivity of the apparatus.
The intensity of the static magnetic field is determined by various factors. One of the factors is the size of a magnetic gap. For the purpose of enhancing the static magnetic field intensity, it is necessary to reduce the magnetic gap formed by magnets and pole pieces opposed to each other with a gap interposed therebetween to the extremity depending on the size of the subject. In addition, members irrelevant to the generation of the static magnetic field but required to be disposed in the magnetic gap needs to be reduced in thickness.
The reason for this is that as the magnetic gap is smaller and the thicknesses of the members disposed in the magnetic gap are thinner, the distance between the subject and the magnets for generating the static magnetic field is reduced, and the intensity of the magnetic field applied to the subject is increased.
Gradient magnetic field coils are disposed in the magnetic gap. The gradient magnetic field coils are coils required to generate a gradient in the static magnetic field created by magnets. During imaging, the gradient magnetic field coils give positional information to the NMR signal generated from the subject, such as a human being or an animal.
As a type of the gradient magnetic field coil, a type in which a flat coil is stacked in layers is available. The gradient magnetic field coil is formed of three layers of coils for applying magnetic fields in the X-direction, Y-direction and Z-direction (static magnetic field direction) with respect to the direction of the static magnetic field.
Japanese Patent Application Laid-Open No. 2004-255182 discloses a flat coil that is used as a gradient magnetic field coil. In the case that the flat coil is stacked in layers to form a gradient magnetic field coil, a flat coil constituting one layer of the gradient magnetic field coil is provided with a groove in the surface portion of a bobbin having a flat plate shape. An electric wire, such as a copper wire, is embedded in the groove and wound in a spiral shape, thereby formed into a spiral electric wire. This spiral portion is referred to as a coil winding portion. Since the coil winding portion is formed into a flat shape, one end of the winding portion is close to the outer edge of the bobbin, but the other end of the winding portion is placed inside the coil winding.
Hence, the other lead wire (drawn-out wire) connected to the coil winding is required to stride over the coil winding from the inside of the coil winding when the lead wire is drawn out to the outside of the coil. The portion striding over the coil winding is herein referred to as a crossover wire. Since the crossover wire is a portion of the lead wire, the thickness of one layer of the flat coil is not just the thickness of the bobbin, but the thickness of the bobbin plus the thickness corresponding to the diameter of the crossover wire (see FIGS. 1a and 1b of Japanese Patent Application Laid-Open No. 2004-255182).
Since the gradient magnetic field coil is formed of three layers of coils, the thickness of the gradient magnetic field coil is the thickness of the three bobbins plus the thickness corresponding to three times the diameter of the crossover wire.
A prior art for reducing the thickness of a flat coil, such as a gradient magnetic field coil, is disclosed in Japanese Patent Application Laid-Open No. 2007-307034. More specifically, a conductive wire is disposed in a groove formed in a mold member in which the groove is formed along a predetermined pattern, an adhesive sheet is placed on the mold member, and the adhesive sheet is made pressure contact with the wire. The wire is removed from the molded member together with the sheet, whereby a sheet, to the surface of which the wire formed into the pattern is secured, is obtained. The sheet and the wire are then alternately stacked, whereby a thin flat coil can be obtained.